[SRU][J][PATCH 2/2] UBUNTU: [SAUCE] shiftfs: fix -EOVERFLOW inside the container

[Alexander Mikhalitsyn]

BugLink: https://bugs.launchpad.net/bugs/1990849

We haven't supported idmapped layers with shiftfs and moreover, that makes
no sense. Once lower fs support idmapped mounts when shiftfs is not needed.

Starting from linux-image-5.15.0-48-generic users started seeing EOVERFLOW
errors from the userspace side on a trivial fs operations inside the containers.

This is caused by patches ("fs: tweak fsuidgid_has_mapping()"),
("fs: support mapped mounts of mapped filesystems"). These patches extends
and enables idmapped mounts support in Ubuntu kernel, but the problem is
that shiftfs was not properly ported.

See also:
("namei: prepare for idmapped mounts")
https://lore.kernel.org/all/20210121131959.646623-15-christian.brauner@ubuntu.com/
("overlayfs: do not mount on top of idmapped mounts")
https://lore.kernel.org/all/20210121131959.646623-29-christian.brauner@ubuntu.com/
as a reference.

This patch should be appied on top of kinetic/master-next and based on the
changes by Andrea Righi 4c934edc66 ("UBUNTU: SAUCE: shiftfs: always rely on init_user_ns")

This commit together with 4c934edc66 ("UBUNTU: SAUCE: shiftfs: always rely on init_user_ns")
have to be ported to the jammy tree too.

Fixes: d347e71d2c0 ("UBUNTU: [SAUCE] shiftfs: support kernel 5.15")
Reported-by: Thomas Parrott <thomas.parrott at canonical.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn at canonical.com>
---
 fs/shiftfs.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/fs/shiftfs.c b/fs/shiftfs.c
index a21624c529f0..5944d6310dca 100644
--- a/fs/shiftfs.c
+++ b/fs/shiftfs.c
@@ -631,10 +631,10 @@ static int shiftfs_rename(struct user_namespace *ns,
 	struct inode *loweri_dir_old = lowerd_dir_old->d_inode,
 		     *loweri_dir_new = lowerd_dir_new->d_inode;
 	struct renamedata rd = {
-		.old_mnt_userns	= ns,
+		.old_mnt_userns	= &init_user_ns,
 		.old_dir	= loweri_dir_old,
 		.old_dentry	= lowerd_old,
-		.new_mnt_userns	= ns,
+		.new_mnt_userns	= &init_user_ns,
 		.new_dir	= loweri_dir_new,
 		.new_dentry	= lowerd_new,
 	};
@@ -971,7 +971,7 @@ shiftfs_posix_acl_xattr_set(const struct xattr_handler *handler,
 		return -EOPNOTSUPP;
 	if (handler->flags == ACL_TYPE_DEFAULT && !S_ISDIR(inode->i_mode))
 		return value ? -EACCES : 0;
-	if (!inode_owner_or_capable(ns, inode))
+	if (!inode_owner_or_capable(&init_user_ns, inode))
 		return -EPERM;
 
 	if (value) {
@@ -2015,6 +2015,16 @@ static int shiftfs_fill_super(struct super_block *sb, void *raw_data,
 		goto out_put_path;
 	}
 
+	/*
+	 * It makes no sense to handle idmapped layers from shiftfs.
+	 * And we didn't support it properly anyway.
+	 */
+	if (is_idmapped_mnt(path.mnt)) {
+		err = -EINVAL;
+		pr_err("idmapped layers are currently not supported\n");
+		goto out_put_path;
+	}
+
 	sb->s_flags |= SB_POSIXACL;
 
 	if (sbinfo->mark) {
-- 
2.34.1